Many structures useful in microelectronic mechanical system (MEMS) devices are produced by layering one or more layers of a material onto a substrate layer, with each such layer possessing potentially different thermo-mechanical properties. For example, small, flat mirrors (also known as micromirrors) used in some MEMS devices are formed by layering a reflective metal film (such as gold or aluminum) onto a silicon substrate layer. The different layers of these mirrors may have significantly different coefficients of thermal expansion (CTEs). Due to this difference in CTEs, such mirrors will typically exhibit a change in their geometrical form (e.g., bow, twist, etc) in response to a change in temperature. This change in form is directly attributable to the stresses that result when the joined layers expand/contract at different rates.
For example, FIGS. 1a and 1b show a three dimensional view and a cross-sectional view, respectively, of a prior art layered structure 101 such as, for example, a micromirror used in optical networking devices. One layer 103 of a reflective material (e.g., gold) with one coefficient of thermal expansion (CTE) is disposed on a substrate layer 102 of another material (e.g., silicon) with a second, different CTE. As the temperature of the structure changes, the difference in CTEs causes a different rate of expansion or contraction (depending upon whether the temperature rises or falls, respectively) of the two layers 103 and 102 relative to each other. Stresses result along the surface 104 where the two layers are joined causing the geometric form of the structure to change (e.g., bend or twist). Geometric form change, as used herein, is defined as any change in the geometric form of the structure that causes the geometric form of the reflective layer of material to detrimentally deform from a desired form. Such geometric form deformation, exemplified by the bending in FIG. 1c, is often undesirable.
In many situations, it is desirable to be able to control or even prevent the geometrical form change that results from the aforementioned stresses. One currently used method of preserving the flatness of micromirrors, illustrated in FIGS. 2a and 2b, compensates for the aforementioned stresses by symmetrically disposing a layer of metal onto each side of the silicon substrate. In this structure, a first layer 203 of a material (e.g., gold) is disposed on one side of a substrate. A second layer 204 of the same material as layer 203 is disposed on the opposite side of substrate 202. In principle, the stresses along layer 206 in FIG. 2b where the substrate 202 is joined with layer 203 will be counterbalanced by the stresses along layer 205 in FIG. 2b where the substrate 202 is joined with layer 204. Therefore, in theory, the stresses that result from the differences in CTE would not lead to the deformation exemplified in FIG. 1c. Such a structure, in theory, would experience identical stresses on each side of the substrate when a temperature change occurs. Therefore, the stresses developed upon a change would not result in a change in the geometric form of the structure.
However, this stress-compensation method has substantial drawbacks. Manufacturing the layered structures of FIGS. 2a and 2b can be difficult, requiring precise control over the physical properties of layers 203 and 204. Variation in, for example, the thickness, density or homogeneity of these layers, which are, for example 10 to 100 nanometers in thickness, could result in unequal stresses between the two layers and the substrate 202 and, as a result, could cause a geometric form change, such as that exemplified in FIG. 1c. Even if the physical properties of the two layers are identical, other problems can arise over time. For example, the stresses induced between layers 203 and 204 and the substrate layer 202 during a temperature variation could cause, over a period of weeks or even months, a change in the crystalline structure of one or more of the layers in the structure. This change, in turn, can lead to a variation in the stresses between layers 203 and 204 and the substrate 202. An imbalance between the counterbalancing stresses on surfaces 205 and 206 in FIG. 2b will result and the geometric form of the layered structure will change in geometric form. In the example of multi-layered micromirrors, this cause of change in geometric form is of particular concern because it may occur after an optical device has been placed in operations. Thus, great expense and time are often involved in removing the device from operations and then identifying and correcting the problem.
Therefore, there remains a need to provide a multilayer micromirror structure that exhibits substantially no form change as a result of a given change in temperature.